Open main menu

Exothermic

Key Stage 3

Meaning

An exothermic process is one that gives out energy. This usually causes surroundings to increase in temperature.

About Exothermic Processes

Most chemical reactions are exothermic with means they release energy to the environment and this is observed by an increase in temperature.
ExothermicSketchGraphKS3.png
The energy stored in the reactants is released in the chemical reaction making the material increase in temperature. The products now have less energy than the reactants.
Freezing, Condensing and Depositing are exothermic changes because they release energy when they happen. The material has less energy after they have happened. However, there is usually no increase in temperature because the material is usually cooled to change state because the surroundings are colder than the material during this process.

Key Stage 4

Meaning

An exothermic process is one that gives out energy. This usually causes surroundings to increase in temperature.

About Exothermic Processes

Foundation

Exothermic reactions usually require energy to begin. This is called the activation energy.
In an exothermic process the potential energy stored in the products is less than the potential energy stored in the reactants.
ExothermicSketchGraphKS4.png
To start the reaction an activation energy is needed. This is usually achieved by initially heating the reactants. Once the reaction starts the energy stored in the reactants is released in the chemical reaction making the material increase in temperature. The products now have less energy than the reactants.

Higher

In a chemical reaction energy is needed to break the chemical bonds holding the atoms together.
In an exothermic reaction energy is then released as atoms form new chemical bonds.
In an exothermic reaction more energy is released in the formation of new bonds in the product than is taken in in the breaking of bonds in the reactants. So there is a net output of energy.

Examples

Higher

Example 1

StructuralDiagramMethane+Oxygen.png
In the reaction between Methane and Oxygen the chemical bonds in the reactants must be broken first before the bonds in the products are formed.
Bond Energy in kJ/mol
C-H 413
O=O 498
O-H 464
C=O 799

There are 4 C-H bonds and 2 O=O bonds.

4 x 413 + 2 x 498 = 2648kJ

Therefore 2648kJ/mol are needed to break the bonds in the reactants.

There are 2 C=O bonds and 4 O-H bonds.

2 x 799 + 4 x 464 = 3454kJ

Therefore 3454kJ/mol is released when the bonds in the products form.

Once the reaction is complete 806kJ will be released.

Example 2

StructuralDiagramHydrogen+Nitrogen.png
In the reaction between Hydrogen and Nitrogen the chemical bonds in the reactants must be broken first before the bonds in the products are formed.
Bond Energy in kJ/mol
H-H 436
N≡N 941
N-H 391

There are 3 H-H bonds and 1 N≡N bonds.

3 x 436 + 1 x 941 = 2249kJ

Therefore 2249kJ/mol are needed to break the bonds in the reactants.

There are 6 N-H bonds.

6 x 391 = 2346kJ

Therefore 2346kJ/mol is released when the bonds in the products form.

Once the reaction is complete 97kJ will be released.

References

AQA

Exothermic reaction, pages 173-5, 178-81, 211,216-7, GCSE Chemistry; Student Book, Collins, AQA
Exothermic reactions, page 134, GCSE Biology; Third Edition, Oxford University Press, AQA
Exothermic reactions, page 165, GCSE Combined Science Trilogy; Biology, CGP, AQA
Exothermic reactions, page 179, GCSE Biology, CGP, AQA
Exothermic reactions, page 61, GCSE Biology; The Revision Guide, CGP, AQA
Exothermic reactions, pages 106, 231-2, GCSE Combined Science Trilogy 1, Hodder, AQA
Exothermic reactions, pages 112-113, 114-119, GCSE Chemistry; Third Edition, Oxford University Press, AQA
Exothermic reactions, pages 129-30, GCSE Chemistry, Hodder, AQA
Exothermic reactions, pages 152, 154, 155, 157, 159, 182-184, GCSE Combined Science Trilogy; Chemistry, CGP, AQA
Exothermic reactions, pages 177, 179, 180, 182, 184, 214-216, GCSE Chemistry, CGP, AQA
Exothermic reactions, pages 61-63, 72, 73, GCSE Chemistry; The Revision Guide, CGP, AQA
Exothermic reactions; reaction profiles, pages 233, 235, GCSE Combined Science Trilogy 1, Hodder, AQA
Exothermic, page 36-7, GCSE Biology; Student Book, Collins, AQA
Exothermic; reaction profiles, page 131, GCSE Chemistry, Hodder, AQA

Edexcel

Exothermic reactions, page 170, GCSE Biology, Pearson, Edexcel
Exothermic reactions, page 270, GCSE Biology, CGP, Edexcel
Exothermic reactions, pages 118, 252, 258-259, GCSE Combined Science, Pearson Edexcel
Exothermic reactions, pages 134, 136, GCSE Combined Science; The Revision Guide, CGP, Edexcel
Exothermic reactions, pages 138, 144-145, GCSE Chemistry, Pearson, Edexcel
Exothermic reactions, pages 243, 244, 248-250, GCSE Chemistry, CGP, Edexcel
Exothermic reactions, pages 83, 85, GCSE Chemistry; The Revision Guide, CGP, Edexcel
Exothermic reactions; equilibrium, page 239, GCSE Combined Science, Pearson Edexcel
Exothermic reactions; equilibrium, page 95, GCSE Chemistry, Pearson, Edexcel

OCR

Exothermic reactions, page 20, Gateway GCSE Biology; The Revision Guide, CGP, OCR
Exothermic reactions, page 41, 42, Gateway GCSE Chemistry; The Revision Guide, CGP, OCR
Exothermic reactions, page 41, Gateway GCSE Biology, Oxford, OCR
Exothermic reactions, pages 102-103, 105, 186, Gateway GCSE Chemistry, Oxford, OCR
Exothermic reactions, pages 18, 111, Gateway GCSE Combined Science; The Revision Guide, CGP, OCR